This invention relates to grain-oriented magnetic steel sheets or strips which are extensively used to make magnetic shields and cores in transformers, generators, and motors. The present invention also relates to a process for manufacturing such oriented steel sheets.
Oriented steel sheets are soft magnetic materials that have a crystallographic orientation in which the {110}&lt;001&gt; orientation, generally referred to as the Goss orientation, is dominant. They have excellent excitation and core loss characteristics in the rolling direction.
A typical process for producing oriented steel sheets comprises the steps of hot-rolling a slab of steel containing about 3.0% Si to obtain a hot-rolled sheet and then cold-rolling the hot-rolled sheet one or more times to attain a final sheet thickness, either immediately after hot rolling or after annealing the hot-rolled sheet. Intermediate annealing is conducted between successive stages of cold rolling. The sheet is then subjected to a continuous decarburization annealing to cause primary recrystallization, followed by application of a parting agent for preventing fusion or seizure, winding the sheet in a coil, and further performing finish annealing at a very high temperature of 1100.degree.-1200.degree. C. The purpose of the finish annealing is two-fold; it is conducted to cause secondary recrystallization, thereby forming a textured structure in which integration in the Goss orientation is dominant, and it is also conducted to remove the precipitate, called an "inhibitor", which has been used to cause secondary recrystallization. The step of removing the precipitate is also known as "purification annealing" and may be regarded as an essential step for obtaining satisfactory magnetic characteristics.
One major disadvantage of oriented magnetic steel sheets produced by the method described above is their extremely high cost since the production process involves special steps such as continuous decarburization annealing and finish annealing at extra-high temperatures of at least 1100.degree. C.
Various R&D efforts have been made with a view of solving this cost problem. For example, the present inventors developed an oriented magnetic steel sheet chiefly characterized by comprising 0.5-2.5% Si, 1.0-2.0% Mn, 0.003-0.015% sol. Al, up to 0.01% C and 0.001-0.010% N, as well as a process for its production that did not need decarburization annealing but which was capable of low-temperature annealing (Japanese Published Unexamined Patent Application No. 1-119644/1989). That process is anticipated to make a great contribution to reducing the cost of oriented magnetic steel sheets by omitting the step of continuous decarburization annealing while lowering the temperature for finish annealing.
Due to an ever-growing societal demand for energy conservation, there is a strong impetus to reduce the core loss of oriented magnetic steel sheets.